I would like to be tagging along with the professor on some "expedition" and you remind him where he is going again (Lake, Forest, or Mountain) and that determines the starter, who you see injured while on the expedition, you both bring it to the lab and he says it likes you or something and you keep it.

Then, your rival busts into your house saying his mom/dad bought him a pokemon (the one your weak to) This will help push the whole (your choosen, your rival is not) approach.